This is really neat. It's a mass produced module for use in fridges and freezers that controls the duration of the start winding current when the compressor starts.
Traditionally the same task would have been done by a current operated switch that would close with the high initial current of the stationary motor and then drop out once the motor was running and the current dropped, but this method achieves the same results in a cheap, simple way with no moving parts.
Here's how it works. It goes in series with the start winding of the motor, which is usually only energised briefly at startup. When power is applied the device appears as a low resistance allowing current to flow through the start winding. The current causes the PTC element to heat up and its resistance rises, which means it dissipates more power, gets hotter and this behaviour results in a sudden increase in resistance until the only current that is passing is the amount required to maintain it at its holding temperature.
When the power to the motor goes off the PTC cools down and returns to its low resistance state.
The thermal effect means that the PTC module will take about a minute to recover to its starting resistance, but in a fridge or freezer the time between compressor cycles is long enough to allow a full reset.
The trip time will depend on the modules initial resistance, starting temperature and the load current passing through it. With a 100W lamp on 240V the current was low enough to take about 90 seconds to final reach its trigger temperature. With very low currents the unit might never trip. This opens up the interesting possibility of wiring one in series with a light fitting so it could only be used with low power lamps.
There are probably many other uses where a suitable load could be protected from long run times or overload. That could be to protect a high power solenoid or other coil, or to protect a motor from a stalled condition.
Best thing is that because they are a mass produced item they are very cheap on eBay in a range of different start resistances. Common resistance values seem to be 4.7, 12, 15, 22 and 33 ohm. I'd guess lower resistances are for higher power loads. There are also a few case styles and pin numbers. I'd recommend the 2 or 3 pin units for ease of connection. Note that an unused pin will probably be connected to the other on the same side and should be covered if not used.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313.TR0.TRC0.A0.H0.Xptc+start+relay.TRS2&_nkw=ptc+start+relay&_sacat=0&LH_TitleDesc=0&_sop=15&_osacat=0&_odkw=ptc+start+relay+3&LH_BIN=1Also keep in mind that these should be regarded with the same mild suspicion applied to all cheap eBay components.
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